Adulterous
by Fleurdelys13
Summary: "It's not cheating if he's out first," she reminds herself. "I am not a cheater." A 'When Addison Cheated On Derek' Fic. Rated T for occasional language and semi-mature themes. R&R!


**_"It's not cheating if he's out first," she reminds herself. "I am not a cheater." _A_ 'When Addison Cheated On Derek' _Fic.**

**So in this fic, let's just pretend that Derek doesn't walk in on them. That may come later. For now they've done it more than once before he finds out. Just for the record. I'm not exactly one for smut. In my mind, Mark and Addie wasn't just a one time thing. Rated T for sometimes language, but only sometimes. R&R and Enjoy!**

* * *

It still hasn't started to snow by the time she gets home after work. The weatherman had called for three inches of snow by seven o'clock, and yet here she is. It's nine and, just like her husband, the snow has just seemed to disappear.

It wouldn't be so bad if it wasn't today. She had reminded him last night, and then in the morning, but he was in such a hurry to get to work that he couldn't even wait for her to eat breakfast, though she knows that it was just another ploy to avoid another ten-minute car ride with her. There's really nothing like solitude to start off the morning.

She'd like to say that she's used to it, that she doesn't expect him to be home when she is, that it doesn't hurt anymore, but she can't. She's _not_ used to it, and it _definitely_ still hurts. Every time he pages her to say that he won't be home, every time he _doesn't_, she breaks a little inside. And she's been breaking an awful lot these last few months.

The only one who remembered was Mark, who even stopped her in the hall to give her a coffee - one milk, two sugar - and wish her a "Happy birthday, Addie." He promised to call her later to check in, but she knew even then not to rely on promises, not even Mark's. It's something she learned long ago.

But for once, a promise is kept in the Shepherd household. Just after nine-thirty, the phone rings, and for once it isn't her absent, workaholic husband to tell her that he's taking another patient, that the ER's overflowed, when she knows for a fact that it _isn't_. The number is one she's known by heart for more than a couple of years and she dries her tears - though it does nothing for her voice - and picks up the phone with a hello as stuffed as her nose.

Mark sees right through her. "Addie? Have you been crying?" he asks.

"No," she insists. "Allergies."

He ignores the fact that she's allergic to nothing and decides to give her a break. It's her birthday. "Is Derek home?"

"Yeah, uh he's just in the bathroom. He's home."

"Is he really?"

"Surgery," she sighs. "Not a big deal, I guess."

"Not a big deal," he repeats, his voice half a whisper that she knows means he doesn't agree. The line is silent for a moment, just the sound of the football game that he had been talking about all day. "Do you want me to come over?"

"No, watch the football game. You've only been talking about it all day. I wouldn't want to keep you."

"Addison," he sighs. "You shouldn't be alone on your birthday. Come on, I can get a cake and candles. How many this year, the usual twenty four?"

She manages to choke out a laugh and tells him once again to stay home, that Derek should be home soon, but even he knows that soon means they've got a few hours at least. "I'm coming over anyway, Add." he replies. "And don't bother locking me out, because I know there's a key under the doormat. I'll see you in twenty."

Nineteen later, he knocks lightly and opens the door, an ice cream cake tucked under his arm and a full bottle of wine in his hand. She doesn't have to get up and stays wrapped tight in the blanket until he brings her over a fork and a plate of cake complete with a glass of Chardonnay. He knows her oh too well, though, and brings the entire bottle over as well. She's somewhat of a compulsory drinker.

One full hour of wine later and she's downed an entire bottle (not without help from Mark), along with the remnants of the one of red left in the fridge. The football game plays in the background, but he muted it and turned on her favorite song. He only knows its her favorite because he was the Derek stand-in at Katharine and David's wedding two years back and the DJ played it while they were dancing, and she just _had_ to drunkenly exclaim how much she loved it. She does the same now, draping her arms around his neck and stumbling around the living room until she finds her balance enough to step on his toe. She has enough sense to jump back fast enough, but her impaired sense of direction just causes her to fall onto the couch where he's sitting. She stays in place for a moment, content to stay where she is, just to be held by someone for once. Derek hasn't so much as kissed her in at least a week.

His right hand rests dangerously high on her thigh, his left behind her opposite knee. She hasn't looked at him, looked at anyone, like this in so long that it feels almost foreign to her. "Add," he whispers. "He doesn't deserve you."

"_Mark_," she sighs. Her hand comes up to caress his face, her fingers barely grazing his rough, unshaven stubble.

"I love you, Addison," he tells her. One slim eyebrow raises, but if she's surprised she doesn't show it. She just stares at him, not surprised at how he feels but that he said it. He's Derek's best friend, his best man, and she's his adulterous wife.

"I'm _not_ a cheater."

"It's not cheating if he's out first." It's obviously the words he lives by, what he does, assuring women that they're not cheaters when he lures them to his bed. Only this time it's her bed, and he's not luring. And somehow, impossibly, it's enough to convince her.

Her lips meet his first, their tongues clashing together in a dance that feels all to familiar. It's familiar because it's their dance, her and Derek's, only Mark's just that much better at it. She doesn't know how it took eleven years to realize that it was she and Mark who were meant to be, not she and Derek. Because sitting here, it all just feels right. The hole in her heart Derek left her is filled and for the first time in a while, Addison Montgomery is happy.

* * *

He wakes up before she does, but he doesn't nudge her or kiss her or anything loving like that. Because they're not a couple. They aren't together. She and Derek are _married_. And he just slept with her. He just slept with his best friend's wife. Talk about kid with the cookie jar.

He knows for a fact that they didn't lock the door. Derek's got a key but they didn't lock the door. It shouldn't matter but it does, because it's the little things that count. The night here or there with the woman he drunkenly admitted to loving after she stepped on his toe. The smile from the nurse he ditched to make sure Addison wouldn't be alone to tell him that she gets it. True love. He wishes he got it.

But in a way, he does. He gets that _he_ went to Katharine and Beckett and Rosa's weddings because Derek had a surgery and he didn't want her to be alone. He gets that he spends more time with her than without, that _he_ sent a celebratory pizza to their house the night that she got her first promotion because Derek was on call for the third night in a row. _He_ remembers birthdays and anniversaries and weddings and dates and parties and this is all he gets: one night with her. One night whose existence they'll both deny for the rest of their lives because if Derek found out - well, he's not sure exactly what he'd do but it'd be bad.

They've only been asleep for half an hour but he knows that Derek could get back at any moment, that he could very well walk in on them and see his best friend in bed with his wife. So he reluctantly nudges her and whispers, "Get up," because if they don't change the sheets then he'll know. He'll know exactly what happened while he was at work and after years of women, Mark knows that it's the only thing worse than actually walking in and seeing it yourself.

She gets in the shower and he doesn't make any remark about joining her because he knows that it's not the night, that they both know they're adulterous. He changes the sheets to a clean new set and hopes Derek won't notice in the two hours he's home at night, even though he knows that he won't. Addison did say something about him sleeping on the couch.

He cleans up the wine and puts the remnants of the melted cake in the freezer, and tops it all off by fluffing the pillows. If Derek's not sleeping in bed he might as well be comfortable. The football game is just ending, and he's not sure who won but he could really care less. It all seems unreal. He just fucked his best friend's wife. He knows it said somewhere in heir vows not to do that. He's not even married and he already broke the vows.

He knows Derek will find out eventually. That he'll come to Mark's apartment and punch him in the face, or kick him in the balls, or shun him forever, and he wants to think that she was worth it, but she really wasn't. They were friends, really good friends. He was the one she told everything to, the one she called after a bad day, or to gloat about the promotion she finally got, or the surgery she pulled off. He's not even sure that she and Derek talk anymore because based on her attitude all night she was expecting to spend her birthday alone.

He sits down and starts flipping through channels because he feels like if he leaves and she's alone for just five minutes on her birthday he'll never hear the end of it. It'll be her bribery and blackmail and it's really not worth an extra moment of his time. Especially when it means one more moment with her.

Not ten minutes after he sits down Derek walks in, coated in snow and seemingly surprised to see him, but only barely. "I didn't want her to be alone," he explains before he can start asking questions that Mark can't answer.

"Thanks." Derek nods at him. "What'd you guys do?"

Mark shrugs. "Watched some football, she even pretended like she knew what was going on. Got a cake too, it's in the freezer. It was pretty melted though, you might want to wait a bit." He shuts off the tv and tosses the remote onto the cushions. "I should get going, I guess."

"Where _is_ Addison?" Derek asks finally.

"Shower." Derek gives him a strange look. "She got kinda drunk, spilt the wine all over herself. We got it out of the couch pretty easily but it was all over her. I think she threw the shirt out." He grabs his coat and pulls it on because it's finally snowing for her birthday, something she's wanted since she was thirteen, she told him. "Derek?"

"Yeah?"

"Wish her a happy birthday, alright?"

His friend looks back at him and cocks his head to the side. "That's today, right. I thought it was next week. Okay, thanks."

_He_ remembers her birthday, and Derek gets her anyway.

And again he's thinking,_ He doesn't deserve you._

* * *

He does find the cake in the freezer, just as melted as Mark had described and he can only wonder why it was left out so long. _They were drunk_, he reminds himself. _Mark Solan did not just sleep with my wife._

Only he _did_. And Derek knows it, too. He knows the look of shame that crosses Mark's face when he sleeps with someone he shouldn't have - they've been best friends for years, after all - and there was a definite shamefulness to his eyes when he walked out.

He checks the laundry hamper and, yes, there's dirty, adulterous sex-infested sheets in the wash right now. And the shirt that he mentioned Addison threw out (which he knows to be just like his wife) is nowhere to be found. Only a dress she forgot to pick up that resides right in the doorway.

The shower shuts off and he heads back out to the kitchen, not eager to tell her that he already left. He decides against cutting her another piece of cake because half of it's already gone and instead lights one single candle and brings it to their room. She's just finished changing into a pair of sweats and a tank top when he comes in, protecting the flame with his spare hand, and brings it to her. It's red just like her hair, just like her favorite color the entire time they were in med school, and she can't help but wonder if that was intentional or a lucky coincidence. "Happy birthday, Addie," he grins, just like Mark said it, and tells her to make a wish.

She blows out the candle and thinks, _I wish I could do tonight all over again_. She doesn't know why but it's the first thing that comes to her mind. Would she still have pretended to be okay with him working late? Would she have gotten as drunk as she did? Would she have slept with Mark? Probably.

But wishes don't come true.

"What'd you wish for?" he asks. _Mark_, he knows.

She laughs. "I can't tell you. Then it won't come true." She kisses him softly on the cheek and takes the candle from him to return to the box in the kitchen. Mark had promised candles but he'd never brought them.

Derek just stands there, completely baffled at her current demeanor. _Maybe they didn't chea__t_, he thinks. _Maybe I'm going crazy. Maybe everything is just coincidental_. But he knows Mark, and he knows Addie. He knows their type. They're the type to cheat on their best friend, on their husband, on whoever as long as they're not around. And he just knows it's not a coincidence, that he's not jumping to conclusions, because there was an unmistakable hickey on the side of her neck, shown off by her wet hair held up in the towel, one he knows for a fact _he_ didn't put there.

She heads off to bed soon enough, their new, clean bed with their new, clean, unadulterous sheets, and he decides to wait a little longer before crashing on the couch. He's not in the mood to sleep with her in that bed anytime soon. Instead he takes out one candle from the box and lights it with a match. It's black, the color of impurity and of sex before marriage, or sex out of marriage, and she had insisted on their wedding day that nothing be black. She wasn't adulterous. At least not then.

He blows it out, not even bothering to make a wish because everything is so crappy that he couldn't think of anything he'd want more than his wife, and not even a wish is going to do that.

The flame quivers and then extinguishes, a metaphor for what he knows has happened to their marriage these past two months. He knows he's been absent a little, missed a couple weddings, forgotten a few birthdays or anniversaries, but he's not to blame here. Neither of them are. As long as he keeps thinking that, he can pretend tonight never happened and move on. _It only happened once. It won't happen again._

He lets out a puff of air that diffuses the thin stream of smoke rising from the wick and standing in the darkness, whispers, "Happy adultery to me."

* * *

** Thanks for reading! This may be more than just a one-shot depending on the reviews I get.**


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